Method and device for ascertaining a clamping force of a braking unit of a motor vehicle

The present invention relates to a method for ascertaining a clamping force of a braking unit of a motor vehicle, the motor vehicle including at least one rotatably mounted wheel and a braking system including at least one braking unit and at least one electric motor, the braking unit including a brake element connected in a rotationally-fixed manner to the wheel and at least one brake body pressable against the brake element, the electric motor including a motor winding and a rotatably mounted rotor, the rotor being coupled by a transmission unit to the brake body in such a way that a clamping force may be generated by a rotation of the rotor, by which the brake body is pressed against the brake element, the rotor being rotatable by applying an electrical motor current to the motor winding, a rotation angle of the rotor and/or a displacement travel of a displaceably mounted element of the transmission unit being ascertained, and a level of the generated clamping force being ascertained as a function of the rotation angle and/or the displacement travel.

In addition, the present invention relates to a method for operating a motor vehicle.

Furthermore, the present invention relates to a device for ascertaining a clamping force, including an evaluation device.

A motor vehicle typically has multiple rotatably mounted wheels. To reduce a travel velocity of the motor vehicle, the motor vehicle generally includes a braking system including at least one braking unit. The braking unit is associated with one of the wheels of the motor vehicle and is designed to generate a friction braking torque, by which a present rotational speed of the wheel is reduced. For this purpose, the braking unit includes a brake element connected in a rotationally-fixed manner to the wheel, for example, a brake disc, and at least one brake body pressable against the brake element. If the brake body is pressed against the brake element, the friction braking torque is thus generated. The braking system preferably includes multiple braking units, each of the braking units being assigned to a different one of the wheels.

Braking systems more and more frequently additionally include an electric motor, in particular including a multiphase motor winding and a rotatably mounted rotor, for actuating the braking unit. The rotor is coupled by a transmission unit with the brake body in such a way that a clamping force may be generated by a rotation of the rotor, by which the brake body is pressed against the brake element. The rotation of the rotor is thus converted by the transmission unit into a displacement of the brake body. A level of the generated clamping force corresponds to a level of the friction braking torque. The higher the clamping force is, the higher the friction braking torque is also. The rotor is rotatable by applying an electric motor current to the motor winding.

Methods for ascertaining the level of the generated clamping force are available in the related art. The knowledge of the clamping force enables an advantageous regulation of the electric motor. Ascertaining a rotation angle of the rotor and/or a displacement travel of a displaceably mounted element of the transmission unit is described in the related art. The rotation angle and the displacement travel correspond to the level of the clamping force. The greater the rotation angle or the displacement travel is, the greater the generated clamping force is also. The level of the clamping force is then accordingly ascertained as a function of the rotation angle and/or the displacement travel.

A method according to the present invention may have the advantage that the accuracy with which the level of the generated clamping force is ascertained is increased. It is provided for this purpose according to an example embodiment of the present invention that a level of the motor current is ascertained, and the level of the clamping force is ascertained as a function of the level of the motor current. In transmission units subject to slip, solely considering the rotation angle and/or the displacement travel results in inaccuracies in the ascertainment of the level of the generated clamping force with increasing absolute rotation angle or displacement travel. These inaccuracies are at least partially compensated for by the additional consideration of the level of the motor current. The level of the motor current also corresponds at least in certain operating points of the electric motor to the level of the generated clamping force. For example, the clamping force increases with an increase of the motor current. The motor current is preferably detected or measured. This is also to be understood as ascertaining the motor current. A sensor unit is thus provided, which is designed to detect the level of the motor current. Alternatively thereto, a parameter is preferably detected or measured which corresponds to the level of the motor current, for example, an electrical motor voltage of the motor winding. The level of the motor current is then ascertained as a function of the detected parameter.

According to one preferred specific embodiment of the present invention, it is provided that the level of the clamping force is ascertained as a function of a first characteristic curve, which describes the clamping force as a function of the rotation angle or the displacement travel. The generated clamping force is ascertainable precisely on the basis of the first characteristic curve. A large number of rotation angles or displacement travels may each be associated with a corresponding clamping force with the aid of the first characteristic curve.

According to an example embodiment of the present invention, the first characteristic curve is preferably changed as a function of the ascertained level of the motor current. A consideration of the level of the motor current which is simple with respect to evaluation technology is achieved in this way. For example, a slope of the first characteristic curve is changed as a function of the ascertained level of the motor current.

According to one preferred specific embodiment of the present invention, it is provided that, as a function of the level of the motor current, on the one hand, and the ascertained rotation angle, on the other hand, a corrected rotation angle is ascertained, the level of the clamping force being ascertained as a function of the corrected rotation angle and/or that, as a function of the level of the motor current, on the one hand, and the ascertained displacement travel, on the other hand, a corrected displacement travel is ascertained, the level of the clamping force being ascertained as a function of the corrected displacement travel. The originally ascertained rotation angle or the originally ascertained displacement travel is thus corrected as a function of the ascertained level of the motor current. For example, a correction factor is ascertained as a function of the level of the motor current and the ascertained rotation angle or the ascertained displacement travel is multiplied for correction by the correction factor. The ascertainment of the level of the clamping force is then accordingly based on the corrected rotation angle or the corrected displacement travel. For example, the level of the clamping force is ascertained as a function of the corrected rotation angle or the corrected displacement travel with the aid of the first characteristic curve.

According to one preferred specific embodiment of the present invention, it is provided that, as a function of the level of the motor current and a second characteristic curve, which describes a clamping force transmission of the braking unit as a function of the level of the motor current, a current-based clamping force is ascertained, the first characteristic curve being changed as a function of the current-based clamping force, and/or the corrected rotation angle and/or the corrected displacement travel being ascertained as a function of the current-based clamping force. Due to the use of the second characteristic curve, a precise consideration of the ascertained level of the motor current when ascertaining the clamping force is achieved. With the aid of the second characteristic curve, a large number of motor current values may each be associated with a corresponding clamping force transmission. The current-based clamping force can then be ascertained, for example, with the aid of the equation()×Fdescribing the current-based clamping force, Kdescribing the clamping force transmission, and Idescribing the ascertained motor current.

According to an example embodiment of the present invention, it is preferably monitored whether a correction situation is present, the first characteristic curve only being changed as a function of the level of the motor current if the correction situation is present, and/or the corrected rotation angle and/or the corrected displacement travel only being ascertained if the correction situation is present. A correction situation is to be understood as a situation in which it is to be presumed that a desired accuracy increase is achieved in the ascertainment of the level of the generated clamping force by the consideration of the level of the motor current. If the correction situation is not present, the ascertained level of the motor current preferably remains unconsidered in the ascertainment of the level of the generated clamping force.

According to one preferred specific embodiment of the present invention, it is provided that a rotational speed threshold is predefined, if a rotational speed of the rotor falling below the rotational speed threshold is present, it being established that the correction situation is present, and/or a displacement speed threshold is predefined, if a displacement speed of the element falling below the displacement speed threshold is present, it being established that the correction situation is present. It is presumed that the correction situation is present if the braking unit or the electric motor is in a static state, thus when the state of the braking unit and the electric motor no longer changes or only changes slightly. This may be established reliably on the basis of the rotational speed or the displacement speed.

A current change threshold is preferably predefined, upon the presence of a motor current change falling below the current change threshold, it being established that the correction situation is present. As mentioned above, the correction situation is present when the electric motor is in a static state. Accordingly, the presence of the correction situation may also be established reliably as a function of the motor current change of the motor current.

The present invention additionally relates to a method for operating a motor vehicle, the motor vehicle including at least one rotatably mounted wheel and a braking system including at least one braking unit and at least one electric motor, the braking unit including a brake element connected in a rotationally-fixed manner to the wheel and at least one brake body pressable against the brake element, the electric motor including a motor winding and a rotatably mounted rotor, the rotor being coupled by a transmission unit to the brake body in such a way that a clamping force may be generated by a rotation of the rotor, by which the brake body is pressed against the brake element, the rotor being rotatable by applying an electric motor current to the motor winding, a level of the generated clamping force being ascertained, and the motor current being applied to the motor winding in such a way that the generated clamping force corresponds to a predefined setpoint clamping force. In the method for operating the motor vehicle, the level of the clamping force is ascertained by the method according to the present invention for ascertaining the clamping force. The above-mentioned advantages also result therefrom. Further preferred features and combinations of features result from the disclosure herein.

A device according to the present invention for ascertaining a clamping force of a braking unit of a motor vehicle, the motor vehicle including at least one rotatably mounted wheel and a braking system including at least one braking unit and at least one electric motor, the braking unit including a brake element connected in a rotationally-fixed manner to the wheel and at least one brake body pressable against the brake element, the electric motor including a motor winding and a rotatably mounted rotor, the rotor being coupled by a transmission unit to the brake body in such a way that a clamping force may be generated by a rotation of the rotor, by which the brake body is pressed against the brake element, the rotor being rotatable by applying an electric motor current to the motor winding, includes an evaluation device which is specially configured to carry out the method according to the present invention for ascertaining the clamping force when used as intended. The above-mentioned advantages also result therefrom. Further preferred features and combinations of features result from the disclosure herein.

According to one preferred specific embodiment of the present invention, the transmission unit is coupled directly to the brake body. The displaceably mounted element or a further displaceably mounted element of the transmission unit is therefore axially applied directly to the brake body at least upon an actuation of the braking unit with respect to a displacement axis, along which the brake body is displaceable. A hydraulic connection between the transmission unit and the brake body is omitted. For example, the transmission unit has a planetary rolling gear transmission, the displaceably mounted element then being a planetary roller spindle drive. A different braking unit is typically associated with each of multiple wheels of the motor vehicle. A different electric motor and a different transmission unit is then preferably associated with each of the braking units. The evaluation device is then designed to ascertain the clamping forces generated by the various electric motors with the aid of the method according to the present invention.

According to another preferred specific embodiment of the present invention, the transmission unit is coupled to a master brake cylinder of the braking system in such a way that upon application of the motor current to the motor winding, a hydraulic piston displaceably mounted in the master brake cylinder is actuated. The master brake cylinder is fluidically connected to the brake body in such a way that the brake body is pressed against the brake element upon an actuation of the hydraulic piston. The transmission unit is thus coupled indirectly with the brake body according to this specific embodiment.

The present invention is explained in greater detail hereinafter on the basis of the figures.

shows a motor vehiclein a simplified representation. Motor vehicleincludes a front wheel axleand a rear wheel axle. Front wheel axleincludes a rotatably mounted first wheeland a rotatably mounted second wheel. Rear axleincludes a rotatably mounted third wheeland a rotatably mounted fourth wheel.

Motor vehicleadditionally includes a braking system. Braking systemincludes a number of braking units,,, andcorresponding to the number of the wheels. Braking units,,, andare only schematically shown in. A different one of braking units,,, oris associated with each of wheels,,, and. Braking units,,, andare designed to generate a friction braking torque in order to reduce the rotational speed of the wheel with which they are associated. For example, a first braking unitof the braking units is designed to reduce the present rotational speed of first wheel. For this purpose, first braking unithas a brake element connected in a rotationally-fixed manner to first wheeland at least one brake body pressable against the brake element. Braking units,, andcorrespond with respect to their design to first braking unit. Braking units,, andeach also include a brake element and at least one brake body, which is pressable against the brake element.

Braking systemadditionally includes an electric motor. Electric motoris also only schematically shown in. Electric motorincludes a rotatably mounted rotor and a motor winding. The rotor is rotatable by applying an electrical motor current Ito the motor winding.

The rotor is coupled to a transmission unit. Transmission unitincludes at least one displaceably mounted element and is designed to convert a rotation of the rotor into a displacement of the displaceably mounted element. For example, transmission unitincludes a spindle gear for this purpose including a rotatably mounted spindle nut and a displaceably mounted spindle.

Braking systemadditionally includes a master brake cylinder. In the present case, master brake cylinderis designed as a tandem master brake cylinder, so that two hydraulic pistons are displaceably mounted in master brake cylinder. The rotor is coupled by transmission unitto master brake cylinderin such a way that the hydraulic pistons are displaceable by a rotation of the rotor.

Braking systemadditionally includes a hydraulic block. Master brake cylinderis fluidically connected by two input linesandto hydraulic block. Hydraulic blockis in turn fluidically connected by four output lines,,, andto the wheel brake cylinders of braking units,,, and. If the hydraulic pistons are displaced in an actuation direction, a clamping force Facting on the brake body is thus generated by a hydraulic fluid present in lines,,,,, and, by which the brake bodies are pressed against the particular brake elements.

Because the rotor is coupled by transmission unitto the hydraulic piston, the clamping force may be generated by the rotation of the rotor. A level of the generated clamping force then corresponds to a rotation angle p of the rotor and a displacement travel of the displaceably mounted element. The greater rotation angle φ is, the higher generated clamping force Fis also. Furthermore, the level of generated clamping force Fcorresponds to the level of the friction braking torque. The higher clamping force Fis, the higher the friction braking torque is also.

Motor vehicleadditionally includes a device. Deviceincludes a rotation angle sensor, which is associated with the rotor and is designed to detect rotation angle φ of the rotor. Deviceadditionally includes a current sensor, which is associated with the motor winding and is designed to detect the level of electric motor current Iflowing through the motor winding. Moreover, deviceincludes an evaluation device. Evaluation deviceis connected for communication to rotation angle sensorand current sensor, so that detected rotation angle φ and the level of motor current Iare provided to evaluation device. Evaluation deviceis designed to ascertain the level of generated clamping force Fas a function of rotation angle φ of the rotor, on the one hand, and the level of motor current I, on the other hand.

Evaluation deviceis additionally designed to ascertain activation signals for switches of a power electronics unit of electric motorand to activate the switches as a function of the activation signals. Evaluation deviceis designed as a control unit. If a friction braking torque is to be generated by braking units,,, and, evaluation deviceactivates the power electronics unit of electric motorin a regulated manner in such a way that generated clamping force Fcorresponds to a predefined setpoint clamping force.

Electric motor current Iis thus applied to the motor winding in such a way that generated clamping force Fcorresponds to the predefined setpoint clamping force. For example, the setpoint clamping force is predefined by actuation of a brake pedal of motor vehicleby a driver of motor vehicle.

shows another exemplary embodiment of motor vehicle. Motor vehicleshown indiffers from motor vehicleshown inwith regard to the design of braking system. The motor vehicle shown inalso includes braking units,,, and.

Braking systemof motor vehicleshown inincludes a number of electric motorscorresponding to the number of braking units,,, and. Moreover, braking systemincludes a number of transmission unitscorresponding to the number of braking units,,, and. Each braking unit,,, andis associated with a different one of electric motorsand a different one of transmission units.

The rotors of electric motorsare also coupled by transmission unitswith the brake bodies of braking units,,, andin such a way that a clamping force Fmay be generated by an application of a motor current Ito the motor windings of electric motors, by which the brake bodies of braking units,,, andare pressed against the particular brake elements. Transmission unitsare each coupled directly to the brake bodies, thus without an interconnected master brake cylinder.

DeviceA of motor vehicleshown inincludes a number of rotation angle sensorscorresponding to the number of electric motors, a different one of rotation angle sensorsbeing associated with each of electric motors.

DeviceA additionally includes a number of current sensorscorresponding to the number of electric motors, a different one of current sensorsbeing associated with each of electric motors.

Rotation angle sensorsand current sensorsare connected for communication to evaluation deviceA, so that rotation angles φ detected by rotation angle sensorsand motor currents Idetected by current sensorsare provided to evaluation deviceA.

Evaluation deviceA of motor vehicleshown inis designed to ascertain the level of clamping forces Fgenerated by electric motorsas a function of rotation angle p of the particular rotor, on the one hand, and the level of particular motor current I, on the other hand.

Moreover, evaluation deviceA is designed to ascertain activation signals for switches of power electronics units of electric motorsand to activate the switches as a function of the activation signals. Evaluation deviceA is designed to activate electric motorsindependently of one another.

Evaluation deviceA activates the power electronics units in such a way that clamping forces Fgenerated by electric motorseach correspond to the predefined setpoint clamping force, as described above with reference to evaluation device.

shows a first characteristic curve L. First characteristic curve Ldescribes the level of generated clamping force Fas a function of rotation angle φ of the rotor. As is apparent from, clamping force Fincreases with an increase of rotation angle φ.

shows a second characteristic curve L. Second characteristic curve Ldescribes the level of a clamping force transmission Kof the braking units as a function of motor current I. As is apparent from, clamping force transmission Kdecreases with an increase of motor current I.

As mentioned above, evaluation deviceactivates electric motorin such a way that generated clamping force Fcorresponds to the predefined setpoint clamping force. Accordingly, evaluation deviceA activates electric motorsin such a way that particular generated clamping force Fcorresponds to the predefined setpoint clamping force.

An advantageous method for ascertaining the level of generated clamping force Fis explained hereinafter with reference to. For this purpose,shows the method on the basis of a flowchart. The method is described by way of example on the basis of evaluation deviceof motor vehicleshown in. However, evaluation deviceA of motor vehicleshown inis also designed to carry out the method and ascertain clamping force Fgenerated by each of electric motorswith the aid of the method.

In a first step S, rotation angle sensordetects present rotation angle φ of the rotor. Moreover, rotation angle sensorprovides detected rotation angle φ to evaluation device.

In a second step S, current sensordetects the level of motor current I. Moreover, current sensorprovides the detected level of motor current Ito evaluation device.

At least steps Sand Sare carried out continuously, so that a profile of rotation angle φ and a profile of motor current Iare provided to evaluation device.

In a third step S, evaluation deviceascertains a clamping force transmission Kcorresponding to the level of motor current Ias a function of the level of motor current Iwith the aid of second characteristic curve L. As a function of ascertained clamping force transmission K, evaluation devicethen ascertains in step Sa current-based clamping force Fwith the aid of the equation F=K(I)×I.

In a fourth step S, evaluation devicechecks whether a correction situation is present. A correction situation is presumed if braking units,,, andand electric motorare in a static state. For this purpose, evaluation devicepredefines a rotational speed threshold and a current change threshold. Moreover, evaluation deviceascertains a rotational speed of the rotor as a function of the profile of rotation angle φ and a motor current change of motor current Ias a function of the profile of motor current I. Evaluation devicethen establishes that the correction situation is present if the ascertained rotational speed of the rotor falls below the rotational speed threshold and the ascertained motor current change falls below the current change threshold.

If evaluation deviceestablishes in step Sthat the correction situation is not present, the sequence refers to a fifth step S. In fifth step S, evaluation devicethen ascertains the level of generated clamping force Fas a function of rotation angle φ detected in step Swith the aid of first characteristic curve L. The level of motor current Iremains unconsidered.

However, if evaluation deviceestablishes in step Sthat the correction situation is present, the sequence refers to a sixth step S. In sixth step S, evaluation devicethen ascertains, as a function of current-based clamping force Fascertained in third step S, a current-based rotation angle φof the rotor with the aid of first characteristic curve L.

In a seventh step S, evaluation devicecompares current-based rotation angle φascertained in step Sto rotation angle p detected in step Sl. If detected rotation angle φ deviates from current-based rotation angle φevaluation devicethus corrects detected rotation angle φ. Evaluation devicethus ascertains a corrected rotation angle φ. For example, a rotation angle falling below detected rotation angle φ is ascertained as corrected rotation angle φif current-based rotation angle φfalls below detected rotation angle φ.